1. Field of the Invention
The present invention relates to amplifiers and, in particular, to balanced differential amplifier circuits.
2. Description of the Related Art
The use of amplifiers is ubiquitous today. Typically a comparatively small voltage input signal, provided from a source such as a transducer, is provided to the input of an amplifier, which amplifies the input signal to provide a larger-amplitude output signal having the same waveform as that of the input signal, within a certain degree of accuracy. The output signal may be used, for example, for signal processing, or to drive recording equipment, an analog-to-digital converter, or an output device such as a loudspeaker.
One problem that plagues many conventional amplifiers is noise. Noise arises in a variety of ways. For example, in amplifiers implemented as part of an integrated circuit (IC) having a substrate, noise can arise from comparatively noisy sections of the substrate and thus adversely affect the amplifier portions of the IC. Pass-transistor switches turning off in switched-capacitor applications may also produce noise. Noise may be generated when unavoidable parasitics, associated with all silicon ICs, provide numerous paths for unwanted disturbances to couple into the signal path of an analog circuit via the substrate, the power supply rails, the ground lines, and/or even directly from nonideal components. Noise may accompany the input signal if the input lines delivering the input signal from the transducer pick up noise from some source external to the IC. Such noise may come, for example, from a noisy power supply that powers a transducer such as a microphone.
Noise may thus be introduced into the signal path of an amplifier, and be amplified along with the input signal, thus causing the amplified output signal to be a distorted representation of the input waveform. Such disturbances and distortions can accumulate, potentially leading to serious loss in signal-to-noise ratio and dynamic range.
Various types of amplifiers are in use. ICs typically implement amplifiers with one or more operational amplifiers ("op amps"). A conventional single-ended op amp, which has differential input and singled-ended output, may be especially prone to being adversely affected by such noise. Such an op amp has positive and negative differential inputs, and a single output terminal that provides an output voltage with respect to ground.
More complicated amplifier configurations, such as differential output op amp circuits or balanced differential output op amp circuits, are often utilized because of their superior noise resistance characteristics. For example, a differential op amp maintains positive and negative signal paths and provides two differential output terminals rather than having a single-ended output. This can help reduce the impact of noise, such as that produced by parasitic couplings or other sources. For example, if noise is injected into one signal path it is likely that the same or similar noise will be injected into the other signal path. Thus, since the output signal is seen as the difference between the two output terminals, the effect of the noise will be canceled out.
Further improvement is possible if such an analog op amp circuit is not only differential, but also balanced. A balanced differential op amp circuit is realized with dual inverting and noninverting signal paths, in a completely symmetrical layout, such that all parasitic injections couple equally into both signal paths as common-mode signals. The differential nature of these circuits causes these common-mode disturbances to cancel (or at least nearly cancel) such that their impact is reduced significantly. Single-ended, differential output, and balanced differential output op amp circuits are described in further detail in David A. Johns & Ken Martin, Analog Integrated Circuit Design (New York: John Wiley & Sons, Inc., 1997): pp. 280-282, and Kenneth R. Laker & Willy M. C. Sansen, Design of Analog Integrated Circuits and Systems (New York: McGraw-Hill, Inc., 1994): pp. 456-462, which are incorporated herein by reference.
Such balanced differential output op amps are more costly (e.g., in terms of IC "real estate" or design complexity) than single-ended op amps, however. For example, in one implementation (described with reference to FIGS. 5-41, page 458, of the Kenneth R. Laker & Willy M. C. Sansen text), two single-ended op amps are interconnected to implement a balanced differential output op amp. however, since the output of one of the single-ended op amps feeds the input of the second, there is increased processing delay as well as phase difference between the two output signals at high frequencies, since the input signals see different hardware (one sees one single-ended op amp and the other sees two).
Another problem that accompanies some differential output op amp designs is that a common-mode feedback (CMFB) circuit typically needs to be added, as described in the David A. Johns & Ken Martin text, pp. 280-282. The extra CMFB circuitry is used to establish the common-mode (i.e., average) output voltage. Ideally, it keeps this common-mode voltage immovable, preferably close to halfway between the power-supply voltages, even when large differential signals are present. Without it, the common-mode voltage is left to drift, since, although the op amp is placed in a feedback configuration, the common-mode loop gain is not typically large enough to control its value. Such is not the case with differential signals as the differential loop gain is typically quite large. However, the design of a good CMFB circuit is not trivial, and may introduce unwanted complexity and/or cost.